The expression network component covers any apparatus which can send and/or receive data packets in the network. In this sense, network components may, for example, be switches or hubs. A switch is an active network component for connecting individual segments of a network to one another. A hub is likewise an active network component, for example a star coupler. The expression connecting medium is used for components of a network via which data and signals can be physically transmitted between different network components. These include, for example, twisted pair cables, coaxial cables and fiber-optical cables. The expression medium covers not only the pure connecting medium but also its connecting components to the network on the one hand (network components) and to the terminal on the other hand (transmission devices). However, since the connecting media in particular may be affected by possible failures, for example as a result of external destruction, these represent the major focus for the design of a media-redundant connection. If a terminal is connected to the network via two physically independent media (media redundancy), then, if one of the media fails, safe and reliable communication is nevertheless possible with the terminal. Since disturbances to individual media are unavoidable, manufacturing failures can be avoided by means of media redundancy in the case of media disturbances, for example in production operations.
Media redundancy can be achieved in a terminal by, for example, equipping the appliance with two transmission devices each having their own media access controller, which controllers are connected to the network via different connecting media. This configuration can be provided in the terminal, for example in the form of two separate network boards. In this case, conventional network components may be used for the inclusion of a terminal. Furthermore, failure monitoring is possible even into the terminal itself, from application to application, in all network protocol layers separately for the two different network links. However, this has the disadvantage that two network boards are necessarily required in each terminal, and this leads to an increase in the costs of the terminals. Furthermore, in this situation, each terminal must be assigned two media access control addresses (MAC addresses) as well as two Internet Protocol addresses (IP addresses). Failure monitoring and a change between the two connections via the two network boards takes place at a higher network protocol layer, generally at the application layer, and is thus done using computer capacity in the terminal. Failure monitoring at the application layer thus results in the computer power of the terminal being restricted with regard to the actual application. The switching times in the event of a disturbance in one of the media are typically in the region of 100 ms.
Another implemented approach for achieving media redundancy is referred to as the ring redundancy approach. This approach is described, by way of example, in the document “Industrial Ethernet™ startet durch—Switching und 100 Mbit/s in der industriellen Kommunikation” [Industrial Ethernet™ starts by—switching and 100 Mbit/s in industrial communication], K. Glas, 1998. In this case, one or more terminals is or are connected to a ring network. If this network is interrupted at one point, then all of the terminals and network components in the ring network are still connected to one another, such that they are still fully serviceable. In this solution, the terminals each require only one network board with one transmission device and one media access controller. In the event of a disturbance, switching takes place at a so-called second network protocol layer (“layer 2”). At the application layer, no program is required for failure monitoring, although no monitoring from application to application is possible, either. Another disadvantage is that the switching times are in the region of 200 ms. A further disadvantage is that it is not possible to use standard switch components to produce the network. The only switch components which may be used are those which are specially designed for use in a network with ring redundancy.